1. Field of the Invention
The present invention relates to an information detecting apparatus. The present invention is suitable for encoders to be used for fine positioning, dimension measurement, metering, measuring of speed, positional information measurement in survey or the like, and in particular measurement control which requires a resolving power of an atomic order (several .ANG.).
2. Related Background Art
Such encoders of this type conventionally comprised a reference scale which has information relating to a position and an angle and a detection means which moves relative to said reference scales to detect the information relating to a position and an angle. Such conventional encoders were classified into several types including an optical encoder, a magnetic encoder and an electrostatic capacity encodes and the like according to a type of said reference scale and said detection means. However, a resolving power of an atomic order could not be obtained yet by these encoders.
There was also proposed as an encoder capable of obtaining a high resolving power a displacement amount measuring device using the theory of a scanning tunnel microscope (STM) in which an atomic configuration represents a reference scale in Japanese Patent Laid-Open No. 62-209302.
In such a scanning tunnel microscope when a voltage is applied between a conductive sample and a conductive probe and the conductive sample and the conductive probe are moved within a distance of 1 nm or to each other around therebetween so as to flow a tunnel current, a tunnel current starts to run to change itself exponential-functionally according to said distance. When the distance from a surface of the sample which is made of a conductive material is kept constant to perform a two-dimensional scanning by using an acute probe, the tunnel current changes due to an atomic configuration or unevenness of the surface so as to obtain an image of the surface ("Solid state physics" vol. 22 No. 3 1987 pp. 176-186). Therefore, by using a sample having a regular atomic configuration or a periodical unevenness as a reference scale and by utilizing the fact that when a relative positional shift is generated between said reference scale and the probe along the direction of the reference scale, a tunnel current periodically changes corresponding to said positional shift, a displacement amount measuring apparatus having the resolving power of the atomic order on the level of several .ANG. can be arranged.
However, though a crystal grating such as graphite (HOPG or Kish graphite) can be used as such reference scale in the above-mentioned conventional encoders, an actual STM image of graphite is observed as a triangular grating as shown in FIG. 1A and the tunnel current to be detected due to a locus of movement of a probe (indicated by an arrow in FIG. 1A) changes as shown in FIG. 1B. For example, in the case of the locus 7a when the probe is on a convex part of the crystal grating as shown, a signal having an excellent S/N with a large amplitude such as a waveform 7a' can be obtained. However, in the case of the locus 7b which is along lower parts (valley) in the grating, an amplitude of a signal is very small and its S/N is deteriorated, such as a waveform 7b'. Further, in the case of the locus 7c which is inclined to a direction of alignment of the crystal grating, a signal has large amplitude on portions mounting on convex parts and small amplitude on portions crossing lower parts as a signal 7c', which results in an output error.
When the displacement amount measuring device is actually arranged as the above-mentioned conventional example, it is desirable that a locus of the probe should pass the locus 7a in FIG. 7A. However, it was very difficult with a reference scale using a crystal grating that a probe and the reference scale should be held not to relatively shift from each other in a perpendicular direction (called the "Y direction") to a direction of detection of an amount of relative movement (called the "X direction") at .ANG. order because there would be caused a temperature drift in a state in which said device is incorporated in the apparatus as a displacement amount measuring device, stress relaxation, a relative shift because of external vibration or the like, non-linearity of a relative movement support mechanism, an error of an angle between the direction of relative movement and the direction of the crystal grating of the reference scale, etc.
In the Patent Publication EPO 304893 A2 (which is corresponding to the U.S. Ser. No. 07/235,552) previously filed by the owner of the present application, there are separately disclosed an apparatus which vibrates a probe in a direction of relative movement between two objects so as to detect an amount and a direction of the relative movement between the probe and a medium in the relative movement direction, and an apparatus which vibrates a probe in a perpendicular direction to a relative movement direction between two objects so as to detect an amount and a direction of the relative movement between the probe and a medium in the perpendicular direction.